This invention relates generally to apparatus for sensing light beams and, more particularly, to fiber optic sensing apparatus for detecting the location of a laser beam.
In recent years, with the development of relatively low cost lasers, control systems utilizing such lasers have come into increasing usage. These control systems have proven particularly beneficial in construction, agricultural and general surveying applications, where a precise alignment of various instruments and materials is frequently essential.
When such a laser control system is utilized outdoors under bright sunlight or at long ranges, the light beam is not ordinarily visible to the human eye, so special light sensors are utilized to detect the precise location of the beam. In the past, such a sensor typically included an array of phototransducers, in the form of phototransistors or silicon photocells, disposed in the region where the light beam was expected to be located. Such sensors have not proven entirely satisfactory, however, because the phototransducers are susceptible to sunlight and radio-frequency interference and because large arrays can be excessively expensive.
Laser beam control systems have been particularly useful in controlling the elevation of the cutting blades of earth moving machines such as trenchers and graders. Such a system typically includes a rotating laser beam transmitter, which is fixed in relation to the field on which work is to be done, and which produces a light beam that rotates in a plane having a predetermined angle relative to a horizontal axis. The plane of light is thus spaced a predetermined distance above the desired elevation of the cutting blade, wherever the blade is located in the field. A laser beam sensing apparatus is mounted on the earth moving machine and senses the elevation of the plane of light relative to the cutting blade. Appropriate signals for use in manually or automatically adjusting the elevation of the cutting blade are then produced.
Typical laser beam elevation sensing apparatus that have been available prior to the present invention include a vertically oriented post on which an array of phototransducers are arranged. The post is disposed so as to be periodically intersected by the rotating beam of light. If a phototransducer near the top of the post senses the periodic light impulses or "blips", it is determined that the cutting blade of the earth moving device is too low, and, conversely, if a phototransducer near the bottom of the post senses the light impulses, it is determined that the cutting blade is too high.
A drawback to such apparatus arises when the earth moving machine changes its direction relative to the transmitter. The phototransducers can become oriented in such fashion that they are directed away from the transmitter and thus are unable to detect the rotating light beam. To overcome this drawback, the prior apparatus required special controls operable whenever there was a failure to detect the rotating laser beam, for rotating the post to seek and locate the beam.
Another drawback to the aforedescribed laser beam sensing apparatus derives from exposure of the phototransducers to radio-frequency interference from such sources as high-tension lines, radar, and ignition systems. Still another drawback to the apparatus arises from the limited speed of the phototransducers, especially when they are fully illuminated by the rotating laser beam. This necessarily limits the range at which the apparatus is operable, because the duration of each of the periodic light impulses that are to be detected varies inversely with the distance from the transmitter. Accordingly, when relatively large areas are to be covered by the earth moving machine, the laser beam transmitter must be frequently repositioned and readjusted.
More recently, laser beam sensing apparatus including arrays of phototransducers arranged in four vertical rows at 90.degree. spacing around the post have been developed. This provides a more omnidirectional sensor, whereby loss of detection of the rotating laser beam does not occur when the earth moving machine changes direction. However, the apparatus nevertheless operates at a substantially lower efficiency when the post is oriented such that the phototransducers receive the laser beam at oblique angles. Additionally, such apparatus are still subject to the aforementioned drawbacks of being susceptible to radio-frequency interference, and of having a limited range. Further, since many more phototransducers are utilized, the omnidirectional apparatus is substantially more expensive.
It will be appreciated from the foregoing that there is a definite need for improved apparatus for sensing the location of a light beam, which is substantially immune to radio-frequency interference, and which is operable at an improved range, while doing so with a minimum number of costly phototransducers. Additionally, there is a definite need for apparatus that can sense, wih a substantially uniform efficiency, the location of a light beam received from any direction. The present invention fulfills these needs.